A fundamental goal of automotive heating, ventilating, and air conditioning (HVAC) systems is to detect and avoid windshield/window fogging conditions.
In an attempt to measure and control the many variables that affect fogging, modern automotive HVAC systems have many sensors and control actuators. A typical system might have a temperature sensor inside the cabin, one measuring ambient temperature outside and others measuring various temperatures of the system internal workings. The occupant may have some input to the system via a set point or other adjustment. Additional sensors measuring sun heating load, humidity, etc. might be available to the system. The set of actuators might include a variable speed blower, some means for varying air temperature, ducting and doors to control the direction of air flow and the ratio of fresh to recirculated air.
It falls to the controller to sort out the range of possible conditions, determine what is needed to detect and avoid windshield/window fogging conditions and coordinate the control of the set of actuators available. This multiple input, multiple output control problem may not fall into any convenient category of traditional control theory. Such considerations such as occupant comfort and vehicle economy may also be considered. The response of the system as well as the relationship between system variables and desired performance, for prevention, may not be linear. Also, it is important to note that despite all the actuators and variables available for control, there may exist conditions under which fog prevention may not be achievable.
Due to practical considerations of size, energy consumption, cost and the wide conceivable range of conditions that automobiles are exposed to, the system plant may simply not be able to supply what is needed. All these considerations lead to a control problem that is far from what is usually encountered in traditional control theory.